I'll add that I typically just use fmt.Sprintf("%v", x), which gives the
shortest possible unambiguous
string representation of the floating point value x.
On Wednesday, August 13, 2025 at 11:33:40 PM UTC+1 Jason E. Aten wrote:
> Hi Jochen,
>
> I think you can get what you want simply with the fmt package.
>
> Consider fmt.Sprintf("%0.5f", x), for example, if you wanted 5 decimal
> places.
>
> If you need to actually round a floating point number, rather than just
> displaying
> a certain number of places, the CockroachDB guys wrote a good summary of
> the approaches here, back in 2017:
>
> https://www.cockroachlabs.com/blog/rounding-implementations-in-go/
>
> As they mention, there is now https://pkg.go.dev/math#Round in the
> standard
> library, which would typically be used by:
>
> multiplying by a power of 10, math.Round()-ing, and then dividing by
> the same power of 10.
>
>
>
>
> On Wednesday, August 13, 2025 at 2:42:18 PM UTC+1 robert engels wrote:
>
>> A floating point number cannot represent all possible real numbers - so
>> when you “shift it back” the number you expect may not be possible.
>>
>> See this on why 0.1 cannot be represented.
>> https://how.dev/answers/why-does-01-not-exist-in-floating-point
>>
>> On Aug 13, 2025, at 7:55 AM, Jochen Voss <joche...@gmail.com> wrote:
>>
>> Dear Robert,
>>
>> Thank you for your response. To make sure I understand: Are you saying
>> Claude's Round() function does not work? (For which example?) Or are you
>> saying it is impossible to do better than Claude's function?
>>
>> Many thanks,
>> Jochen
>>
>> On Wednesday, 13 August 2025 at 13:52:08 UTC+1 robert engels wrote:
>>
>>> Read up on numerical analysis - what you are asking for is impossible :)
>>>
>>> You need to convert to a string, or use BCD/fixed place values - like
>>> github.com/robaho/fixed
>>>
>>> On Aug 13, 2025, at 7:42 AM, Jochen Voss <joche...@gmail.com> wrote:
>>>
>>> Dear all,
>>>
>>> I would like to define a function "func Round(x float64, digits int)
>>> float64" which rounds to the given number of digits, in the sense that
>>> I want "strconv.FormatFloat(x, 'f', -1, 64)" to show at most the given
>>> number of digits after the decimal point.
>>>
>>> The following naive approach does not work:
>>>
>>> func Round(x float64, digits int) float64 {
>>> eps := math.Pow10(-digits)
>>> return math.Round(x/eps) * eps
>>> }
>>>
>>> For example for rounding math.Pi to five digits I get "3.1415900000000003"
>>> instead of "3.14159". https://go.dev/play/p/gRtHG6ZgTjj .
>>>
>>> Anthropic's Claude suggested the following:
>>>
>>> func Round(x float64, digits int) float64 {
>>> if digits <= 0 {
>>> pow := math.Pow10(-digits)
>>> return math.Round(x/pow) * pow
>>> }
>>>
>>> format := "%." + strconv.Itoa(digits) + "f"
>>> s := fmt.Sprintf(format, x)
>>> result, _ := strconv.ParseFloat(s, 64)
>>>
>>> return result
>>> }
>>>
>>> This seems to work, but also seems quite inefficient.
>>>
>>> Is there a better way?
>>>
>>> All the best,
>>> Jochen
>>>
>>> PS.: Here is some testing code for experimenting
>>> https://go.dev/play/p/Xcd6fTvYend
>>>
>>>
>>>
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>>>
>>>
>>>
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